The invention concerns an aid to navigation in a sequential system of radio-navigation by phase reception which is used in differential mode of operation, such as the system known under the name OMEGA, and which is envisaged for general application on a terrestrial scale.
It is known that the differential-mode use of a radio-navigation system presumes supplementary transmission of phase corrections defined by the difference between each phase received at a selected site and the corresponding theoretical phase. The theoretical phase at a point is that which, in the theoretical network of position lines, satisfies a reciprocal and stationary correspondence with a geographical position of this point. The received phase varies complexly with time in relation to the theoretical phase, and this produces untimely errors in this reciprocal correspondence. It is therefore apparent that the knowledge of the corrections thus transmitted permits substantial reduction of these untimely errors.
This differential usage applies only in a certain zone close to the selected site. It is therefore desirable that the supplementary transmission should be extended to this zone, whatever type of receivers of the sequential radio-navigation system are to be found there.
For the same reason, a large number of supplementary transmissions of this kind also need to be provided if the differential-mode radio-navigation system is desired for use over long distances. In particular, in coastal areas the supplementary phase-correction transmissions should be made in sufficient number to enable the differential-mode radio-navigation system to be used along all coast and thereby obtain the advantage of greater precision.
The radio-navigation system receivers intended for additionally receiving this correction transmission can, for their part, range from the ordinary type to the highest degree of precision according to the application. Also it is clearly necessary for the same correction receiver to be usable over great distances for a full set of these supplementary transmissions and without involving the user in complicated operations. In other words, an intervening normalization is required over the set of supplementary phase correction transmissions.
Each correction transmitting station has therefore to allow general diffusion over a certain zone for receivers which range from the ordinary type to the greatest precision; more generally, a large number of such transmitting stations have to satisfy this criterion, while at the same time permitting a normalization of utilization and great precision.
In the case of the Omega system, for example, eight base transmitting stations are provided to cover all the globe, the distance between transmitting stations here being of the order of 8000 km (5000 miles).
According to present estimates, the utilization range of one correction transmitting station will at most be several hundred kilometers. It is therefore clear that a considerable number of correction transmitting stations is required.
Having regard to the present congestion of the ether, frequency allocations are becoming increasingly more difficult to obtain for differential radio-navigation, even with technical requirements as fundamental as those which have just been presented.
In the future it seems unlikely that the allocation of new radio channels will be sufficient to solve this problem, for it will probably be necessary to install progressively more and more correction transmitting stations, and this will quickly saturate these new channels.
Moreover it is clear that most of the channels already occupied are not compatible with all the requirements set out above for the supplementary transmission of corrections, namely, diffusion over predetermined zones, great simplicity of utilization, normalization over a large number of different transmissions and high precision.